The present invention relates to an X-ray exposure apparatus for printing a fine pattern such as a semiconductor integrated circuit pattern or the like. More particularly, the invention is concerned with an optical system including a reflecting mirror and suited advantageously for scanning an object with an X-ray beam in an X-ray exposure apparatus which uses synchrotron radiation as a source.
In recent years, synchrotron radiation (hereinafter abridged to SR) becomes a promising source for the X-ray exposure apparatus. The SR is electromagnetic radiation generated by high energy electrons which run along a circular orbit at a velocity close to that of light. The SR is emitted in the direction tangential to the orbit and features excellent directivity and high intensity in the wavelength range of about 0.5 to 1.5 nm usually used in the X-ray exposure apparatus. It is however observed that although the SR is emitted within an angular range of several tens milliradians in the horizontal plane (orbital plane of electrons), the divergence angle of the component whose wavelength is useful for the X-ray exposure is at most 1 milliradian in a vertical plane (the plane extending orthogonally to the orbital plane and in parallel to the plane of the drawing in the following description). Accordingly, in order to expose a fixed object placed at several to ten meters from the SR light source over a square field having a side of several tens millimeters, the object must be scanned in the vertical direction of deflecting the SR.
To this end, a rotatable reflecting mirror is used in a SR beam scanning system known heretofore, as disclosed in "Proceeding of SPIE - The International Society for Optical Engineering", Vol. 448, pp. 93-101. FIG. 2A of the accompanying drawings shows an arrangement of this known optical system in a plane orthogonal to the orbital plane of electrons. The beam 12 emitted from a SR source 11 is deflected by a reflecting mirror 13 to a mask 14 and a wafer 15 held with a gap of several tens .mu.m therebetween, whereby the pattern of the mask is printed on the wafer 15. For convenience' sake, the combination of the mask and the wafer will collectively be termed an object in the following description. The reflecting surface of the mirror 13 is implemented in a cylindrical form ("U" form) to collimate the in the horizontal direction, as illustrated in FIG. 2B, wherein the mirror 13 is so placed that the center axis of the cylindrical surface lies in the plane parallel to that of the drawing, as viewed in FIG. 2A. The reflecting mirror can be rotated about, the axis 16 extending perpendicularly to the plane of the drawing within an angular range .DELTA..alpha. of about .+-.3 mrad relative to the center position where the grazing incidence angle .alpha. (i.e. angle formed between the incident beam and the reflecting surface) is 24 mrad. As a result, the object located at about 7 m from the reflecting mirror is scanned with the beam over a width of about 40 mm. On the other hand, in the horizontal plane (orthogonal to the plane of the drawing), the SR beam is collimated by a concave reflecting surface, whereby a beam width of about 40 mm is obtained on the object.
In another scanning system known heretofore, the reflecting mirror is translated (i.e. moved parallelwise), as disclosed in JP-A-60-226122 filed in the name of the present assignee and laid open on Nov. 11, 1985 and JP-A-61-276223 filed in the name of Fujitsu, Ltd. and laid open on Dec. 6, 1986. More specifically, in the case of this known scanning system illustrated in FIGS. 3A and 3B of the accompanying drawings, a reflecting mirror 23 is moved in the same direction as the direction in which a SR beam 22 radiated from the source 21 travels and the direction perpendicular thereto for allowing an object 24 to be scanned with the beam. The reflecting mirror surface is implemented in a cylindrical form ("U" form), as is illustrated in FIG. 2B, the center axis of which lies in the plane parallel to that of the drawing.
In the first mentioned prior art, the angle of incidence at the reflecting mirror varies by .DELTA..alpha. during the scanning operation. In this connection, it should be however noted that in general the reflectivity, in the soft X-ray region varies significant by dependence with the angle of incidence. As a consequence, the beam intensity varies along with the position on the object, giving rise to a first problem that line width control in the resist pattern becomes difficult. Additionally, the direction of the incident beam on the object varies within the range of 2.DELTA..alpha., involving a second problem that the printed pattern undergoes distortion when compared with the exposure using collimated beam. The first mentioned problem may be solved by controlling the rotating speed of the reflecting mirror as a function of the angle of incidence so that the uniform illumination is obtained. However, in view of the fact that the reflectivity varies with time due to contamination on the reflecting surface, the uniform illumination can not always be assured. Besides, the second mentioned problem still remains unsolved.
With the second mentioned prior art system, it is certainly possible to make constant both the beam intensity and the incident direction over the object. However, a greater difficulty will be encountered in implementing a translation mechanism with the grazing incidence angle being kept constant, as compared with the mechanism for the rotational scanning.